You ask, they reassure you, you feel better — and a day later the doubt is back. If reassurance keeps wearing off, there is a reason, and it is one you can do something about.
Short answer: reassurance soothes the immediate feeling, while the deeper uncertainty underneath it stays put. So the relief is real and temporary — the worry settles, then returns, and you find yourself needing proof again. This is a loop rather than a personal failing, and loops can be understood and gently interrupted.
In anxious attachment, uncertainty about closeness can feel genuinely threatening, so the mind looks for certainty to make the threat go away. A partner saying "of course I love you" delivers a hit of that certainty. But permanent certainty about another person is beyond what anyone can hold — relationships are always, at some level, uncertain — so reassurance is being asked to do an impossible job: make the uncertainty stop forever. The comfort was real; it fades because it was carrying a task too large for it.
Needing reassurance is human, and it says little about whether the relationship is failing. More reassurance, delivered more often, rarely fixes the loop either, because the loop was never really about the supply. And on the other side, a worry that keeps returning is sometimes pointing at a real inconsistency worth looking at — so a returning doubt deserves a look rather than a dismissal.
Notice the difference between reassurance that supports you and reassurance that carries you. Everyone needs comfort; the aim is enough self-trust that you can sit with uncertainty for a while before reaching for proof. That self-trust is what makes reassurance a support rather than a supply, and it is something that grows.
Go deeper
The Anxious Attached works through the reassurance pattern and what builds more lasting self-trust — practical, and from both sides of the relationship.